SVG (generic ink reader) + Stylus Labs Write (.svg / .svgz)¶

Status: read support, two layers sharing one code base (core/src/inkterop/formats/svg/):

SvgReader (format_id "svg") — a bounded generic subset of SVG 1.1, tuned so that our own SVG writer (render/svg.py) round-trips.
WriteReader (format_id "write") — Stylus Labs Write documents, whose native format is plain SVG (.svgz = the same file gzipped).

Registration order matters: both claim .svg, and reader_for returns the first detect() hit, so WriteReader must be registered before SvgReader. (The .svg writer slot belongs to render/svg.py's SvgWriter, unrelated to these readers.)

Generic reader scope¶

Supported [verified against hand-made fixtures]:

Feature	Notes
`<path>`	commands `M/m L/l H/h V/v C/c Q/q Z/z`; curves flattened with a fixed 16-segment subdivision
`<polyline>`, `<line>`
styling	presentation attributes + inline `style=`: `stroke`, `stroke-width`, `stroke-opacity`, `fill`, `fill-opacity`, `opacity`, `stroke-linecap`, `color` (for `currentColor`)
`transform`	`translate` / `scale` / `matrix` / `rotate` flattened through a stack; `skewX/skewY` log-skipped
nested `<svg x y>`	treated as `translate(x, y)` `[inferred]`
`<text>`	-> `ir.TextBlock`

Ignored by design: <defs>, <clipPath>, CSS <style> blocks, <use>, gradients/patterns/masks/filters, <image>; paths containing A(rc) or any other unsupported command are skipped whole with a logged warning. Shapes with fill but no stroke and no data-rmi-* attributes are skipped — in the wild they are decorations, not ink [inferred].

Units `[inferred]`¶

Page bounds come from the root viewBox (fallbacks: width/height, then content extent). point_scale = (root width converted to pt) / (viewBox width), with unit conversion pt=1, px/unitless=0.75 (CSS 96 dpi), in=72, mm=72/25.4, cm=72/2.54, pc=12.

Round-tripping our own SvgWriter output `[verified by test]`¶

render/svg.py embeds per-stroke data-rmi-tool, data-rmi-pressure, data-rmi-width attributes (space-separated per-point values) plus <g data-rmi-layer> / <g data-rmi-template> groups. The reader:

restores the tool family from data-rmi-tool and PRESSURE from data-rmi-pressure;
STROKED_CONSTANT strokes are plain stroked paths; the WIDTH channel is data-rmi-width re-scaled so its first value equals the rendered stroke-width (the writer's unit scale is not stored) [inferred];
STROKED_VARIABLE strokes were tessellated into a closed filled outline (n forward points, optional 7-point round cap fan, n reversed points, optional fan; n = data-rmi channel length): the centerline is rebuilt as forward/reverse midpoints, per-point WIDTH as forward/reverse distances — geometry verified against .rm fixtures within 0.05 pt;
all-coincident strokes became <circle>: center replicated to the channel length;
data-rmi-layer groups -> ir.Layer; data-rmi-template content is skipped, only the background kind survives (pitch isn't embedded).

Per-point ALPHA and TIMESTAMP/tilt channels do not survive an SVG round-trip (the writer never embeds them).

Stylus Labs Write¶

License bright line: the Write app source (github.com/styluslabs/Write) is AGPL and was not consulted. Facts below come from the styluslabs/templates README (which documents the page structure), and from the bytes of Write-produced documents: styluslabs.com serves its website pages as Write SVGs (corpus/third-party/styluslabs-write/: site1_page002.svg, features_page002.svg, plus template Dot grid 25.svg; provenance in corpus/third-party/MANIFEST.toml).

Structure¶

Fact	Confidence
Multi-page: root `<svg id="write-document">` holding one `<svg class="write-page">` per page	`[verified]` in templates README + template files; site samples are single pages whose root is the page itself
`<g class="write-content write-v3">` carries page setup: `width`, `height`, `xruling`, `yruling`, `marginLeft`, `papercolor`, `rulecolor`	`[verified]` (README + samples; `width`/`height` absent on site samples -> fall back to the page `<svg>`'s width/height)
`rulecolor` is alpha-first `#AARRGGBB` (`#7F0000FF` matches the ruleline's `stroke-opacity="0.498"`); `papercolor` is `#RRGGBB`	`[verified]`
Ruling is drawn as real elements in `<g class="ruleline">` (with `rect.pagerect` page background) — decoration, skipped as content	`[verified]`
Pen strokes: `<path class="write-stroke-pen" fill="none" stroke="#RRGGBB" stroke-width="W" stroke-linecap="round" d="M x y l dx dy ...">` (absolute moveto + relative linetos)	`[verified]`
Handwritten hyperlinks: ink wrapped in `<a class="hyperref">`, and those paths carry no `write-stroke-*` class (site1: 149 classed + 27 unclassed strokes)	`[verified]`
`__comx` / `__comy` per-stroke attrs = center-of-mass bookkeeping, ignorable	`[inferred]`
Stroke classes other than `write-stroke-pen` (highlighter etc.)	`[unknown]` — unobserved; mapped to `ToolFamily.UNKNOWN`
Units = CSS px at 96 dpi -> `point_scale` 0.75	`[inferred]`
Per-point width/pressure storage	`[unknown]` — observed strokes are constant-width; Write may bake dynamics into geometry

IR mapping¶

yruling>0 & xruling>0 -> TemplateBackground("grid"), yruling only -> "lines" (pitch = yruling), xruling only -> "unknown"; ruling gray derived from rulecolor luminance blended by its alpha.
Non-white papercolor (no ruling) -> ColorBackground.
xruling/yruling/marginLeft/papercolor/rulecolor raw strings are kept in page.extra["write"].
write-stroke-<kind> -> NativeTool("write", kind); pen -> ToolFamily.PEN, everything else UNKNOWN.

Fixtures: core/tests/fixtures/svg/ (hand-made, CC0 — write-mini.svg is schema-by-us, not a Write export); real Write samples are corpus-gated tests (tests/test_svg_reader.py).